Bird scavenging of mollusc shells
For most of the past year I’ve been working on a landfill site, excavating and recording archaeological features ahead of the construction of a new landfill cell. To the west of the excavation area I’m working in there is an open landfill tip. Despite the best efforts of the man charged with bird control, the tip often attracts crows and flocks of gulls. Walking around the site over the past few months, I’ve picked up three marine mollusc shells lying on the ground surface. These are not archaeological shells, rather I suspect they have been picked up by birds on the tip and then dropped.
Bivalve shells such as oysters have two halves (called valves), which are hinged. Archaeological assemblages of edible marine shells often contain higher number of one valve than the other (see Claassen 1998, 74; Law & Winder 2009). The differences may be due to processes which occur after burial (known as taphonomic processes), deliberate separation of the valves by people (from example, serving oysters in the cupped valve and discarding the flat valve) or other depletion processes such as reuse of the valves. Bird scavenging of shells from an open dump could be one of these processes, and may account for stray finds of shells of edible species at archaeological sites. It would be nice to be able to identify some marks on the shells which are definitely indicative of bird scavenging, however the shells I found are all quite broken and there is no way to control for other factors which may have caused damage.
I plotted a scale plan of the finds in relation to the edge of the open landfill area. There were also a number of bones close to the scallop shell which I believe are also bird-scavenged. The maximum distance a bird could transport a shell or bone is likely to be quite far, but I wonder if there are clusters around certain distances.
Claassen, C., 1998. Shells. (Cambridge: Cambridge University Press).
Law, M., and Jessica Winder, 2009. Different rates of survival of right and left valves of european oyster (Ostrea edulis L.) from archaeological sites in Britain, Archaeo+Malacology Newsletter 16, 1-3. (PDF)